1. Field of the Invention
This invention relates to a diaphragm as a constructional member for an air spring used in vehicles and various industrial equipment, and more particularly to an improvement of the diaphragm for avoiding the degradation of performances in the air spring.
2. Description of Related Art
The air spring is widely used in large-size buses and trucks at the present because it is possible to fixedly hold a vehicle posture against a load change from curb weight to authorized payload or the like by controlling the supply of air from exterior and the discharge of air to the exterior and also spring constant can be set to a low level to improve the ride comfort and prevent the damage of freight.
As a typical example of the air spring, there is a diaphragm type air spring, which is usual to have a structure as shown in FIGS. 6a and 6b. That is, the air spring comprises an outer sleeve 8 having an air supply and discharge port 8a and an inner sleeve 9 fitting into an opening portion of the outer sleeve 8 and a two-folded diaphragm 10 connecting the outer sleeve 8 to the inner sleeve 9 to form an air chamber A therebetween.
In the air spring having the above structure, when vibration is input from the side of the inner sleeve 9 (piston) to change the inner sleeve 9 from an ordinary state of FIG. 7a to a state of moving in a compression stroke direction as shown in FIG. 7b (at the compression stroke, the pressure inside the air chamber A becomes high and the spring constant becomes high), the diaphragm 10 changes as follows. That is, when the inner sleeve 9 moves in a compression direction, the diaphragm 10 fixed to the inner sleeve also moves in the compression direction together with the inner sleeve 9. In this case, the diaphragm 10 is pulled into an inside of the outer sleeve 8 without increasing so the size thereof. The diaphragm 10 is provided in its inside with two cord reinforcing layers, cords 11a, 11b (nylon cord, polyester cord or the like) of which layers being crossed with each other as shown in FIG. 8, so that the cross cord angle in these reinforcing cord layers varies in the movement of the diaphragm 10 or these cords conduct so-called pantograph movement, whereby it is possible to smoothly change the shape of the diaphragm without creating wrinkles or the like.
In this type of the air spring, a hollow cylindrical body for suppressing expansion of diaphragm in the input of vibrations has recently been attached to the outer sleeve at the outside of the diaphragm in order to compact the volume of the air spring in a radial direction thereof (for space saving).
The arrangement of such a hollow cylindrical body is effective for the space-saving and can ensure the durability of the air spring even if the service pressure is increased. And also, since the durability can be ensured, the number of the cord reinforcing layers in the diaphragm can be decreased from 2 layers usually used to 1 layer. However, the use of the hollow cylindrical body newly causes the following problem. Accordingly, it is strongly demanded to solve this problem.
When the number of the cord reinforcing layers in the diaphragm is decreased from 2 to 1, it is required to arrange the cords constituting the cord reinforcing layer in a direction along an axial direction of the diaphragm before the attachment to the sleeves from a viewpoint of the balance when air is enclosed in the air spring. However, if it is intended to expand the diaphragm by inputting vibration to the air spring, since the size of the diaphragm can not be changed by the pantograph movement of the cords constituting the reinforcing layer, the size deformation is caused by stretching the elastomer (rubber) between the cords to enlarge the pitch of the cord, while if the diaphragm is contracted (size reduction), a force contracting from the produced size is acted to compress rubber between the cords and also air pressure is applied to the inside of the diaphragm, whereby a folding wrinkle is formed in the diaphragm. As a result, there is a problem that the durable life of the diaphragm is considerably degraded due to the repetitive formation of such a wrinkle.
It is, therefore, an object of the invention to provide a novel diaphragm capable of solving the above problem of the conventional technique in the air spring provided with the hollow cylindrical body for suppressing the expansion due to the input of vibration.
According to the invention, there is the provision of a diaphragm for air spring comprising an outer sleeve provided with a hollow cylindrical body suppressing an expansion of a diaphragm at a stroke in compression direction of the air spring and an air supply and discharge port, an inner sleeve fitted into an inside of the outer sleeve and a folded diaphragm connecting the outer sleeve and the inner sleeve to each other to from an air chamber, characterized in that an inner diameter of the diaphragm before the assembling at a region engaged with at least an inner sleeve of the air spring is made equal to or smaller than a minimum outer diameter of the inner sleeve so as to act tensile force in a circumferential direction of the diaphragm at a state of engaging with the inner sleeve.
In the invention, the region of the diaphragm engaging with the inner sleeve is a region maintaining the contact between the inner sleeve and the diaphragm at a maximum stroke in the compression direction of the air spring. And also, the diaphragm is a columnar shape wherein the inner diameter is equal over its full length, or a tapered shape wherein the inner diameter becomes gradually small from a position engaging with the outer sleeve toward a position engaging with the inner sleeve. Further, the diaphragm is provided with a cord reinforcing layer containing cords substantially arranged along an axial direction of the diaphragm.
According to the invention, the inner diameter of the diaphragm before the assembling at the region engaged with at least the inner sleeve of the air spring is made smaller than the outer diameter of the inner sleeve, so that when the diaphragm is assembled onto the inner sleeve, tensile force is acted to the engaged position in the circumferential direction of the diaphragm and hence the compression force created in the contraction of the diaphragm is mitigated by the tensile force and the initial properties of the diaphragm are maintained over a long period of time.